Frequency responses of SAW ladder filters with apodized interdigital transducers and width-controlled reflectors

Frequency responses of surface acoustic wave (SAW) ladder filters with apodized interdigital transducers (IDTs) and width-controlled reflection gratings are investigated in this study. A novel transmission matrix method is employed to simulate the scattering parameters as well as the characteristic impedance of one-port resonators, while the frequency responses of the multi-section ladder filters cascaded from one-port resonators are constructed via the image parameter method. Different window functions such as Kaiser and Chebyshev functions are adopted to adjust the overlap width distributions of the IDTs and electrode width variations of the gratings. Effects on the resonance frequencies and spurious modes of one-port resonators are illustrated. Different stepwise weighting schemes are applied to recalculate the width distributions of IDTs and gratings for the realization of the modulated one-port resonators. Simulation results show that the bandwidth of the ladder filter can be effectively increased using the apodized IDTs. By adjusting the overlap width ratios of the one-port resonators in the parallel and series arms, the sideband suppression level of the ladder filter is enhanced. The results obtained in this study can be employed to design RF filters with broad bandwidth and good sideband suppression.